1. Field of the Invention
The present invention relates to a printing apparatus, and more particularly, to a printing apparatus that is used in an electrophotographic system such as a copying machine and a printer, and includes an optical head having a light emitting element array.
2. Description of the Related Art
As an example of printing apparatus employing an electrophotographic system, there is given a printing apparatus including an exposure head including a light source which has an array of light emitting elements such as light emitting diodes. When the light source having the array of light emitting elements is used in the exposure head, the exposure head itself is downsized. Thus, quietness of the printing apparatus can be easily achieved. As an example of using the light emitting elements such as light emitting diodes as the light source of the exposure head as a component of the printing apparatus, there is given a light emitting element array disclosed in Japanese Patent Application Laid-Open No. H10-55890. The light emitting element array disclosed in Japanese Patent Application Laid-Open No. H10-55890 is manufactured by forming multiple organic EL elements collectively on an insulating substrate such as a glass substrate. Further, in Japanese Patent Application Laid-Open No. H10-55890, light beams output from the light emitting element array are converged with a converging rod lens array. The converged light beams are radiated onto a photosensitive drum so as to form a predetermined image.
However, the single-row light emitting element array disclosed in Japanese Patent Application Laid-Open No. H10-55890 has a problem in that the light emitting element array as a whole is treated as a defective product in a case where any single one of the light emitting elements of the array becomes defective. This is because, when printing is performed with the light emitting element array including the defective element, the defective element causes streak-like image defects on a printed material. The streak-like image defects are significantly conspicuous on the printed material, which are quite unacceptable to users. However, when a defect of even single one of the light emitting elements of the array cannot be accepted, there are problems of a decrease in manufacture yield and an increase in manufacturing cost of the light emitting element array.
As solutions to the above-mentioned problems, there have been proposed a method disclosed in Japanese Patent Application Laid-Open No. 2009-154420, and a method disclosed in Japanese Patent Application Laid-Open No. 2008-65200. In the configuration proposed in Japanese Patent Application Laid-Open No. 2009-154420, light emitting elements in multiple rows are provided to a single line head. When the light emitting elements of the line head are switched in row units, operation can be performed without using rows including defective elements. With this, a high yield can be achieved. Further, in the method proposed in Japanese Patent Application Laid-Open No. 2008-65200, a light emitting portion of a single light emitting element is split into multiple sub light emitting portions. Specifically, in the method proposed in Japanese Patent Application Laid-Open No. 2008-65200, when one of the multiple sub light emitting portions becomes defective, only the defective part is disconnected from a pixel drive circuit. With this, electric current from the pixel drive circuit can be applied only to normal sub light emitting portions. Thus, the light emitting elements each can emit light with desired light intensity.
However, in the light emitting elements in multiple rows of Japanese Patent Application Laid-Open No. 2009-154420, when even single one of the light emitting elements in one row becomes defective, the row including the defective light emitting element is treated as a defective part. This state has no difference from the case of the light emitting element array of Japanese Patent Application Laid-Open No. H10-55890. Thus, when even single one of the light emitting elements in each column becomes defective in all the rows of the light emitting elements of the line head, the rows including the defective light emitting elements are each treated as a defective part. As a result, the line head itself is treated as a defective product. In particular, when even some of the rows of the light emitting elements have a high defect rate, a yield of the line head itself may be insufficient.
Meanwhile, in the method proposed in Japanese Patent Application Laid-Open No. 2008-65200, print image quality may not be maintained as described below. For example, in a case where a single pixel is split into two sub light emitting portions (portion A and portion B) in the method proposed in Japanese Patent Application Laid-Open No. 2008-65200, when the pixel is normal, both the two sub light emitting portions (portion A and portion B) emit light. However, when any of the two sub light emitting portions becomes defective, only non-defective one emits light (only the portion B when the portion A is defective). In this case, in a pixel having such a defective sub light emitting portion, electric current from the pixel drive circuit is applied only to the non-defective sub light emitting portion, and the non-defective sub light emitting portion emits strong light. Also with this, required total light intensity may be cooperatively achieved by all the pixels. However, light emitting areas of the pixels vary from each other depending on whether or not a defect has occurred, and hence unexpected unevenness in color tone appears on a printed material. As a result, there arises a problem of difficulty in maintaining print image quality.